1. Field of the Invention
The present invention relates to a multi-section Roots vacuum pump of the reverse flow cooling type with an internal flow division arrangement. The present invention can be applied to the reverse flow cooling type multi-section Roots vacuum pump which is operated at a high compression ratio in the range between atmospheric pressure and 10.sup.-3 Torr and at a relatively high temperature.
2. Description of the Related Art
Generally in a Roots type vacuum pump in which rotor pairs rotating in a housing to draw in and discharge gas have a minute clearance from the housing which accommodates the rotor pairs therein, it is important that the clearance between the rotor and the housing be as small as possible in order to realize a pump having a high performance. So far, in a multi-section Roots vacuum pump driven at a high compression ratio, wherein the temperature will rise relatively high due to the compression heat during the operation, the selected clearance is usually based on the expected temperature of the rotor pairs and the housing in the operating state. However, since the housing is in direct contact with the external atmosphere, the temperature of the housing is affected by the change of the temperature of the atmosphere. Therefore, the selected clearance should be such that variation of the thermal expansion of the housing due to a change of the temperature of the atmosphere is taken into consideration. This situation is an obstacle to the realization of the as small as possible clearance.
In the prior art, in order to minimize the effect of the ambient temperature and to cool the pump device, a jacket for cooling water is provided at the peripheral portion of the housing, and a thermo-sensor and other devices also provided to control the flow of cooling water running through the jacket and the temperature thereof, to maintain as constant a temperature of the housing as possible. However, this method is not advantageous because the thermo-sensor and/or other devices for controlling the flow and temperature of cooling water are required.
In a prior art Roots type vacuum pump, due to the radiation of heat to the open air, the temperature of the housing becomes lower than that of the rotor pairs inside the housing during the operation of the pump, thus the clearance between the housing and the rotor pairs is reduced because the amount of thermal expansion of the housing becomes smaller than the amount of thermal expansion of the rotor pairs, resulting in contact between the housing and the rotor pairs. To prevent such contact, a jacket is provided at a peripheral portion of the housing of the pump, with an open passage provided to communicate one end of the jacket to the outlet passage, wherein the discharged gas heated by the compression will be circulated within the jacket by the flow turbulence caused by the rotation of the rotor pairs, to equalize the temperature of the housing and that of the rotor pairs. However, because the open passage of the jacket is provided at only one place, and an insufficient amount of discharged gas flows through the jacket, a satisfactory result cannot be attained.
Furthermore, an outer piping is led from one portion of the jacket to the outlet pipe of the pump, to ease the flow of the discharged gas to the jacket. However, in this example, the outer piping is specifically required and, moreover, the pressure difference between the upstream side and downstream side of the outer piping is too small to secure a sufficient flow of gas. Therefore, this method is not advantageous.
Further, Japanese Unexamined Patent Publication No. 59-115489 has disclosed, for a reverse flow cooling type multi-section Roots vacuum pump, a pump that comprises a connection pipe provided to connect the outlet passage of a specific pump section with the inlet passage of the following pump section, a cooler incorporated to the connection pipe, and a reverse flow pipe branched off from the connection pipe at the downstream side of the cooler and arranged to lead the reverse flow cooling gas to the preceding pump section.
In a reverse flow cooling type multi-section Roots vacuum pump, reverse flow pipes are provided to lead reverse flow cooling gas from the connection pipes between adjacent pump sections in a bifurcated manner, as an outside piping arrangement of the pump. However, such an outside piping arrangement has a comparatively complicated structure and, therefore, is relatively unprofitable from the viewpoints of miniaturization of the pump and manufacturing cost of the piping arrangement. Moreover, the reverse flow piping, manufactured from comparatively thin piping material such as flexible piping, can be a source of noise. Further, since the housing comes into direct contact with the open air, and the temperature of the housing will be affected by changes of ambient temperature during the operation of the pump, the predetermined clearance amount between the rotor pairs and the housing must additionally include the amount of change by thermal expansion caused by the ambient thermal change, causing a problem in the realization of a practical smallest clearance. Therefore, preferably the gas leakage from such a clearance is minimized as much as possible, to realize a pump having a high performance.